Magnetic read/write systems such as, for example, tape and disk drives, store digital information on a magnetic storage media by writing magnetic bit patterns to the storage media. The storage media is a ferromagnetic material and the data is stored via magnetic dipoles. A binary one is represented as a change in magnetization within a bit window of the storage media, while a binary zero is represented as no magnetization change within the bit window of the storage media.
Magnetic dipoles are created on the media by altering the magnetic field, or B-Field, established by the write head under control of the write driver circuit. The write head comprises an inductive coil which is wrapped around a conductive head yoke. The write driver circuit is coupled to the write head via the inductive coil. Changes in write current polarity generated by the write driver circuit establish an altering B-field in the inductive coil and in the head yoke. The yoke is comprised of a very magnetically malleable material and readily conducts the induced B-field. The yoke generally is horse-shoe shaped and has opened ends which form a throat at the interface of the yoke and the magnetic media. Because the yoke is open at the media interface, a flinge B-field intercepts the ferromagnetic media and magnetic dipoles are written.
The minimum field strength necessary to flip the magnetic dipoles is commonly referred to as the coercivity field or coercivity bubble. The shape of the bubble determines many properties associated with the written data, such as jitter, density, and signal-to-noise ratio (SNR). Both the write head and the write driver circuit influence the coercivity bubble, and thus, the size and response of the B-field. The optimal coercivity bubble places a vertical field through the entire thickness of the storage media. A smaller bubble may not place a vertical field through the entire thickness of the storage media and, therefore, may not completely magnetize all domains of the storage media. A large bubble increases the data PW50, which is the time between the 50% points on an isolated pulse, and decreases data density. If the bubble is excessively large, data in adjacent tracks could be erased. The size of the bubble is determined by the magnitude of the write current, which is proportional to the B-field. During manufacturing, the head throat height typically is not well controlled and, therefore, utilizing a programmable write current improves head yield. In most magnetic disk and tape drive systems, the write current is programmable via a current analog-to-digital converter (IDAC). This allows the coercivity bubble to be optimized for each write head by varying the magnitude of the write current.
In order to create sharp media transitions, the write current rise time should be less than the media rise time. The optimal write current transition is slightly under damped in order to create optimal media transitions. This type of write current waveform provides the best rise time without creating excessive write current overshoot. Extreme overshoot may erase adjacent data. A write current rise time greater than the media rise time creates jagged domain transitions, which increase data jitter. To control the current overshoot, a damping resistor typically is placed across the write head coil. The amount of overshoot can be controlled by appropriately selecting the value of this damping resistor.
The write current rise time is a function of the write head inductance, the write current, and the supply voltage of the write driver circuit. The rise time can be defined as EQU Trise=K*Iwr*Ind/VDD
where K is a parameter based on process and circuit topology, Iwr is the write current, Ind is the head inductance, and VDD is the supply voltage. The designer of the write driver circuit has some freedom in selecting the design topology and the supply voltage. Media properties dictate the head inductance, Ind, and the write current, Iwr. As seen from the above equation, the write current rise time decreases as the supply voltage of the write driver circuit increases.
Storage density capability of magnetic storage media available on the market has increased greatly over the past several years. In order to take advantage of this increase in storage capacity, the number of write driver circuits integrated in a single read/write system needs to be increased in order to increase the number of write channels that can be utilized by the read/write system. However, in order to increase the number of write driver circuits without exceeding the power consumption requirements of the read/write system, low-power write driver circuits are required. As stated above, low-power write driver circuits (i.e., write driver circuits having relatively low supply voltages) have higher write current rise times than that associated with high-power write driver circuits.
It would be advantageous to be able to optimize the write current rise time for the particular write head being used, especially where the write driver circuit has a low supply voltage (e.g., 3.3-volts), in order to improve overall performance of the write driver circuit without having to increase the supply voltage of the write driver circuit. Currently, write driver circuits do not allow the damping resistance to be programmably varied in order to determine the damping resistance that results in the best write current rise time. Accordingly, a need exists for a method and apparatus for programmably setting and adjusting the damping resistance to optimize the write current rise time for the particular write head being used.